Field of the Invention
The invention relates to a method for checking a three-way exhaust catalytic converter disposed in an exhaust pope of an internal-combustion engine. The engine is operated with an air/fuel ratio that corresponds to a conversion window of the catalytic converter. The NOx concentration in the exhaust pipe downstream of the catalytic converter is recorded by a NOx sensor. The NOx concentration values are used to determine conversion properties of the catalytic converter.
To monitor exhaust catalytic converters for internal-combustion engines, methods that use the oxygen storage capacity (OSC) of the catalytic converter are known. The storage capacity correlates to the hydrocarbon conversion in the catalytic converter. If the catalytic converter has good conversion properties, the lambda fluctuations upstream of the catalytic converter, which are produced by the lambda regulator and are recorded by a first lambda sensor, are smoothed out by the oxygen storage capacity of the catalytic converter. If the conversion properties of the catalytic converter have been reduced or eliminated as a result of aging, poisoning by leaded fuel or by combustion failures, the control fluctuation on the lambda sensor which is present upstream of the catalytic converter makes its presence felt downstream of the catalytic converter. By comparing the signal amplitudes from the two lambda sensors, it is possible to work out that the catalytic converter is functioning correctly or is defective (see Published, Non-Prosecuted German Patent Application DE 23 28 459 A1).
In vehicles which are certified in accordance with the stringent ultra low emission vehicle (ULEV) or super ultra low emission vehicle (SULEV) limits, deteriorations in the conversion rates of even a few percent lead to the diagnosis limits being exceeded. However, relatively low amplitude ratios are determined at these conversion rates. Consequently, there is no longer a reliable distinction between a defective catalytic converter and a correctly functioning catalytic converter, in particular taking account of series-production scatter and the nonlinear correlation between oxygen storage capacity and HC conversion.
German Patent DE 197 14 293 C1 uses the relationship between exothermic energy conversion in the catalytic converter and HC conversion by temperature methods for diagnosing the catalytic converter. To assess the conversion properties of the catalytic converter, a temperature model is used to calculate the thermal energy generated in a reference catalytic converter which is not catalytically coated, and to compare the thermal energy with the thermal energy generated in the catalytic converter, which is measured. A measure for the conversion ability of the catalytic converter is determined from the difference, and by comparing the measure with a comparison value. From this it is determined that either the catalytic converter is defective or that the catalytic converter has sufficient conversion properties.
It is accordingly an object of the invention to provide a method for checking a three-way exhaust catalytic converter of an internal-combustion engine which overcomes the above-mentioned disadvantages of the prior art methods of this general type, which checks the conversion ability of a three-way exhaust catalytic converter which does not require expensive temperature sensors and a second lambda sensor.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for checking a conversion ability of a three-way exhaust catalytic converter disposed in an exhaust pipe of an internal-combustion engine. The internal-combustion engine is operated with an air/fuel ratio corresponding to a conversion window of the three-way exhaust catalytic converter. The method includes using an NOx sensor for recording an NOx concentration in the exhaust pipe downstream of the three-way exhaust catalytic converter; and using values of the NOx concentration to determine conversion properties of the three-way exhaust catalytic converter by performing the below listed steps while the internal-combustion engine is warming up with the air/fuel ratio. A sum value is formed of the NOx concentration measured from a beginning of a starting of the internal-combustion engine until a time at which the NOx concentration falls below a predetermined threshold. The sum value is compared with the predetermined threshold, and the three-way exhaust catalytic converter is determined to be aged if the predetermined threshold is exceeded.
The present invention is based on the idea of using a NOx sensor for determining the NOx concentration downstream of the catalytic converter to be diagnosed in order to check a three-way catalytic converter. The required condition for the diagnosis method is an air/fuel ratio xcex which is within the conversion window of the catalytic converter (typically: 0.995xe2x89xa6xcexxe2x89xa60.999), since the NOx conversion falls considerably in particular at lambda values of greater than 0.999.
On account of the close relationship between the NOx conversion and the HC conversion, the measured NOx concentration can be used to work out not only the directly determined NOx conversion properties but also the HC conversion properties of a three-way catalytic converter. The relationship applies within the air/fuel ratio range described above.
The method has the advantage in particular that it also allows the diagnosis of exhaust-gas installations with three-way catalytic converters with a high degree of accuracy, even if stringent exhaust-gas limit values are to be observed.
In accordance with an added mode of the invention, there is the step of activating a malfunction indication device and/or inputting an entry into a malfunction memory if the predetermined threshold is exceeded.
In accordance with another mode of the invention, there is the step of storing the predetermined threshold in a characteristic diagram as a function of operating variables of the internal-combustion engine.
In accordance with a further mode of the invention, there is the step of using an intake air mass flow rate of the internal-combustion engine, the air/fuel ratio and/or a rotational speed of the internal-combustion engine as operating variables.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for checking a conversion ability of a three-way exhaust catalytic converter disposed in an exhaust pipe of an internal-combustion engine. The internal-combustion engine is operated with an air/fuel ratio corresponding to a conversion window of the three-way exhaust catalytic converter. The method includes using an NOx sensor for recording an NOx concentration in the exhaust pipe downstream of the three-way exhaust catalytic converter; and using the values of the NOx concentration to determine conversion properties of the three-way exhaust catalytic converter by performing the below listed steps while the internal combustion engine is warming up and secondary air is supplied. A sum value is formed from the NOx concentration measured from a time at which a supply of the secondary air is terminated until a further time at which the NOx concentration falls below a predetermined threshold. The sum value is compared with the predetermined threshold, and it is inferred the exhaust catalytic converter has aged if the predetermined threshold is exceeded.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for checking a three-way exhaust catalytic converter of an internal-combustion engine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.